1. Field of the Invention
The present invention relates to a display apparatus and more specifically to a photoluminescent display apparatus which makes an image, that has been formed by modulating excitation light such as ultraviolet rays generated from a light source by using a liquid crystal panel or the like, to be visible by a phosphor screen and displays it.
2. Description of a Related Art
There is a growing use in recent years of liquid crystal displays (LCDs) taking the place of cathode-ray tubes (CRTs) as displays for flat televisions and computers. The LCDs are also being considered for diagnostic by monitoring in medical equipment, such as ultrasonic apparatus, CT apparatus, MRI apparatus and CR apparatus, and also for viewing art works and for use in large information display equipment in airports.
Prior art LCDs, though with their advantages of being small in size and light in weight, have a drawback of a large dependence on an angle of view. That is, when the viewing direction is perpendicular to the display surface, the luminance is maximum. As the viewing direction shifts away from being perpendicular, the luminance and contrast sharply decrease and the tone reverses, so that how the image appears changes depending on the angle of view.
To eliminate the angle-of-view dependence and increase the brightness, a photoluminescent liquid crystal display (PL-LCD) has been developed in recent years. The basic principle of the PL-LCD is that an image formed by modulating excitation light such as ultraviolet rays generated from a light source by using a liquid crystal panel or the like is made visible by a phosphor screen and displayed. The image displayed on the PL-LCD is a fluorescent image similar to that formed on a CRT, and is thus not dependent on the viewing angle. Further, when a color image is to be displayed, three color phosphors need only be coated non-overlappingly as in the CRT, obviating the need for color filters, which in turn is expected to enhance an efficiency. It is known, however, that random directions of excitation light incident on the liquid crystal cells cause fuzziness and a reduced contrast of the image. Japanese Patent Application Publication JP-A-9-511588, which corresponds to WO-A-95-27920, discloses a display screen having a light collecting member in cells of liquid crystal layer in order to improve the PL-LCD.
The criterion in selecting the phosphor used in the photoluminescent display apparatus takes into account only whether the phosphor in question has a high efficiency of excitation by ultraviolet rays or blue light rays. For example, U.S. Pat. No. 5,608,554 discloses a display apparatus which uses three kinds of phosphors that emit three colors, red, green and blue (RGB), respectively, rather than filtering a white beam of light produced by a light source through color filters to disperse light into three RGB colors. Among the phosphors selected for such a use are the ones that have a good emission spectrum when exposed to deep blue excitation light from a backlight that has a main radiation peak in the wavelength range of 380 nm to 420 nm. Further, Journal of Applied Physics 88.4660 (2000) discloses a phosphor having high excitation efficiency at wavelengths of 365 nm and 394 nm.
The excitation efficiency for ultraviolet radiations mentioned above is a concept used in evaluating phosphors and is normally measured by radiating the excitation light against the surface of a phosphor layer thick enough to absorb the excitation light and by observing the luminescence light emitted from the same surface. The excitation efficiency measured by this manner, however, greatly differs from an effective efficiency which is obtained by observing the luminescence light emitted from the side of the phosphor layer opposite to the excitation light irradiated surface. In this arrangement, the relation between the sharpness and the layer thickness also differs from that of the conventional arrangement, and the layer thickness that provides the high sharpness required for the diagnostic using the CR apparatus is, for example, 120 xcexcm or less, which will be described later in detail.
The phosphors used in the conventional PL-LCD have high excitation efficiencies for ultraviolet rays or the like but many of them cannot absorb the excitation light sufficiently. Hence, relatively reducing the thickness of the phosphor layer to realize a high sharpness results in a low energy efficiency when observed from the surface opposite to the excitation light irradiated surface, which poses a problem in putting it into practical use. On the other hand, increasing the layer thickness to raise the energy efficiency results in a reduced sharpness. Thus it is impossible to achieve both the high brightness and the high precision at the same time. Conversely, too high an absorption of the excitation light results in only the shallow surface of the phosphor layer being excited, thus reducing the energy efficiency.
U.S. Pat. No. 4,822,144 discloses a display apparatus in which an interference filter layer that transmits ultraviolet rays but reflects visible rays is provided between the excitation backlight portion and the phosphor layer to reduce the loss of the visible rays of light occurring in the phosphor layer.
In view of the aforementioned problems, it is an object of the present invention to provide a display apparatus such as PL-LCD in which excitation light is radiated against the back of the phosphor layer to produce luminescent light from the front and which can realize an image with high sharpness, high brightness and high contrast with high efficiency.
The contact type display apparatus according to the present invention comprises: a light source unit for generating excitation light having a predetermined wavelength; an optical element for modulating the excitation light generated by the light source unit for each of pixels in a two-dimensional plane; and a fluorescent screen for receiving the excitation light modulated by the optical element at a first surface and emitting visible light from a second surface opposite to the first surface.
The rear projection type display apparatus according to the present invention comprises: a light source unit for generating excitation light having a predetermined wavelength; an optical element for modulating the excitation light generated by the light source unit for each of pixels in a two-dimensional plane; a projection lens to project the excitation light modulated by the optical element; and a fluorescent screen for receiving the excitation light projected by the projection lens at a first surface and emitting visible light from a second surface opposite to the first surface.
According to a first aspect of the present invention, the fluorescent screen includes a layer of phosphor having an absorption coefficient not smaller than 1xc3x97102 cmxe2x88x921 for the excitation light. The phosphor layer should preferably be 120 xcexcm or less thick to enhance the brightness and sharpness. In this application the absorption coefficient refers to an apparent absorption coefficient and a method of evaluating it will be detailed later.
According to a second aspect of the present invention, the fluorescent screen includes a layer of phosphor in which a thickness of the layer of phosphor that gives a maximum brightness caused by the excitation light is not larger than 120 xcexcm. More preferably, the thickness that gives the maximum brightness is 80 xcexcm or less.
According to a third aspect of the present invention, the fluorescent screen includes a layer of phosphor in which a product of an absorption coefficient for the excitation light and a thickness of the layer of phosphor is within a range from 1 to 8. More preferably, the product of the absorption coefficient and the layer thickness is within a range from 2 to 4. It is also desired that the thickness of the phosphor layer be 120 xcexcm or less to enhance the brightness and sharpness.
With the above configurations, in a system that radiates excitation light against the back of the phosphor layer to produce luminescent light from the front of the phosphor layer, because the excitation light is properly absorbed in the phosphor layer to produce visible light, an image with high sharpness, high brightness and high contrast can be obtained with high efficiency.